This project is designed to apply the immunohistochemical assay (ICA) for estrogen (ER) and progesterone receptors (PgR) in a prospective, nonrandomized clinical trial. Previously untreated, elderly women with breast cancer 5.0 cm or greater, and with ER and/or PgR positive tumors by ICA will be placed on an 8 week course of tamoxifen. Such patients are usually treated with some type of therapy before surgery in order to reduce tumor bulk. To determine endocrine response, tumor size and axillary lymph nodes will be measured by caliper and by nuclear magnetic imaging before and after therapy. ICA will be performed on fine needle aspirates (FNA) and/or core needle biopsies on both freshly frozen and formalin fixed, paraffin-embedded tissue. Biochemical ER and PgR on open biopsy specimens will be performed in the laboratory of Dr. William L. McGuire in San Antonio, Texas. All assay results will be correlated with clinical endocrine response in order to determine the procedure with the best sensitivity, specificity and positive predictive value. The ability to perform accurate receptor assays on initial tumor obtained by FNA or core biopsy, especially on routinely processed tissue, should materially reduce false negative assay results such as may be obtained when mastectomy specimens are analyzed. Expenses and logistical problems incurred in obtaining an open biopsy for diagnosis and receptor analysis with freezing, and transport of frozen tissue, would be obviated, since in-patient or ambulatory surgery suites are not needed for FNA or needle biopsy, formalin fixed. Such an assay would also be of use in determining receptor content in women diagnosed with very small lesions, now commonly seen, as well as those with Stage IV disease where adequate amounts of tissue for biochemistry are often unavailable. Furthermore, ICA on routinely processed specimens would permit retrospective assay of archival pathology specimens. This would be of research value and useful for women where no steroid receptor assay was initially performed. Finally, ICAs will be quantified by television imaging and compared with qualitative and semi- quantified results as determined by visual inspection.